Ultraviolet energy absorbed by the human skin can produce an erythemal reaction (redness), the intensity of which is dependent upon the amount of energy absorbed. Ultraviolet radiation from both sunlight and artificial sources has been divided into three bands (UV-A, UV-B, and UV-C) which emit different quantities of energy and therefore produce an erythemal reaction at different time intervals after exposure. The amount of energy from any source required to produce a minimally perceptible redness reaction of the skin is termed the Minimal Erythema Dose or MED.
UV-A radiation is present in the sunlight reaching the earth's surface and has a wavelength of 320 to 400 nanometers (nm.). It can cause tanning of the skin but is weak in causing reddening of the skin. About 20 to about 50 joules/cm.sup.2 of UV-A energy is required to produce one MED. The erythema reaction is maximal in intensity about 24 hours after exposure.
UV-B radiation is present in the sunlight reaching the earth's surface and has a wavelength of 290 to 320 nm. It causes the sunburn reaction which also stimulates pigmentation (tanning) in the skin. Approximately 20 to 50 millijoules/cm.sup.2 of UV-B energy is required to produce one MED (i.e., about 1,000 times less than the dose of UV-A). The erythema reaction is maximal in intensity at from about 6 to about 20 hours after exposure.
UV-C radiation has a wavelength of 200 to 290 nm. and is not present in the sunlight reaching the earth's surface but can be emitted by artificial ultraviolet sources. It is not effective in stimulating pigmentation but does cause erythema requiring about 5 to 20 millijoules/cm.sup.2 to produce one MED.
The tanning ability of an individual is genetically predetermined and is governed by the individual's capacity to produce melanin pigment within the pigment cells when stimulated by UV-B and UV-A. The extent of any erythemal response is a function of skin color and thus less time is required to produce a MED in light skinned individuals than to produce a MED in dark skinned individuals.
The most rapid way to cause tanning is to allow the sun to produce erythema of the skin. Erythema sufficient to induce tanning yet not so severe as to cause pain requires only half the time of exposure that is required to produce a painful sunburn. Suntanning can occur at the UV-A wavelengths but develops slowly under natural conditions. Tanning most commonly develops after exposure to the "sunburn" UV-B band.
Sunscreen preparations are commercially available which extend the time it takes the sun to produce a sunburn. Such preparations contain chemicals which can absorb ultraviolet light at various wavelengths, for example, 4-(dimethylamino)benzoic acid, 2-ethylhexyl ester (Escalol 507) and 2-hydroxy-4-methoxybenzophenone (UVinul M40) which absorb UV-A, or an opaque substance that physically reflects or scatters the ultraviolet light, i.e. talc.
Ultraviolet absorbing compounds and sunscreen compositions containing the same are disclosed, for example, in U.S. Pat. No. 3,004,896; 3,189,615; 3,403,207; 3,479,428; 3,644,614; 3,670,074; 3,751,563; 3,821,363; 3,892,844; 4,514,383; and British Pat. No. 1,291,917.
Commercially available sun screen and sun block formulations provide excellent protection against severe sun burning of exposed skin for extended periods so long as they remain on exposed areas and are not washed off by bathing. Unfortunately, bathing in pool water and ocean water will usually result in most conventional sun screen and sun block formulations being washed away from the skin thereby leaving exposed areas of skin. Attempts have been made at formulating sun screen products which are moisture resistant. For example, see U.S. Pat. No. 3,666,732.
Accordingly, effective sun screen and sun block formulations which remain on the skin even after bathing, that is, are water-resistant, for extended periods of time would indeed fulfill a long-felt want.